The invention relates to an overvoltage protective device to protect electrical onboard power supply systems, in particular, onboard vehicle power supply systems, against overvoltages as well as to a method for operating the overvoltage protective device according to the preambles of the independent claims.
Electrical devices in mobile units, particularly in vehicles, are usually supplied by an onboard power supply system which, in turn, is powered by an electrical generator. When the generator supplies little or no energy to the onboard power supply system, the onboard power supply system receives the necessary energy from an energy store, e. g., a battery. When the generator delivers more energy than is needed in the onboard power supply system or in the loads, the energy store serves as a buffer for this energy. Overvoltages in the onboard power supply system may occur if there is a failure of the energy store acting as a buffer, e. g., due to corrosion of the terminals or in the event of a cable break between the energy store and the generator and the like.
A further possible cause for the occurrence of overvoltages in the onboard power supply system is present if, for example, inductive electrical loads such as, e. g., fan motors or seat adjustments, are switched away from the onboard power supply system while the vehicle is in operation. The load shedding causes an overvoltage in the onboard power supply system with, at times, very high values. Details are cited in a number of references, for which, e. g., DIN (German Industrial Standard) 40839, Part I, is cited as representative.
Such voltage peaks cause considerable problems in the electrical units. Devices, particularly control devices, for the internal combustion engine, for the transmission or for the antilock system, are usually constructed from discrete and/or integrated semiconductor switches which can malfunction as a result of overvoltages and/or can even be destroyed. A measure for the occurrence of irreversible damage is the breakdown voltage of the semiconductors that are used, which is normally between approximately 50 V and 60 V in automotive applications.
German Patent No. 40 39 404 C2 discloses an overvoltage protective device wherein a controllable switch arrangement short-circuits the generator if an overvoltage occurs. The switch arrangement is provided with an integrated MOSFET switch and offers the option of predetermining switching thresholds via comparators. Via this arrangement, an attempt is made to minimize the overvoltage in the onboard power supply system by influencing the exciter winding of the generator. But even this arrangement cannot reliably prevent the occurrence of a voltage pulse, even at a low height. Particularly a possible undesirable voltage rise due to feedback on the part of the loads in the power supply system cannot be compensated.
In conventional onboard power supply systems, so-called suppressor or limiter diodes are frequently used in the electronic devices or, centrally, on the generator. The suppressor diodes are intended to limit the possible overvoltages to a value that is harmless for the semiconductor concerned. But the terminal voltage of the suppressor diodes depends so greatly on ambient influences, particularly on the temperature and the internal resistance, that a wide tolerance range of the supply voltage of the onboard power supply system must be accepted.
Because of the possible voltage peaks that must be considered, semiconductor switches with breakdown voltages of 50 volt and more are applied in 12 V onboard power supply systems that are used at present. Integrated semiconductor components that meet these requirements are expensive and have a large chip surface because of the technology. A large chip surface, however, translates into a high price for an integrated semiconductor component. If it is intended to use higher onboard voltages, this size and cost problem intensifies because of the rising breakdown voltages.
German Patent Application DE 39 15 198 A1, on which the present invention is based, discloses a protective device for the protection of electrical equipment against transient overvoltages, particularly lightning stroke. The overvoltage protective device has a limiting device whose limiting threshold is above the rated voltage of a voltage supply system. A power branch having a voltage detecting or sensing unit and an actuating unit is connected in parallel to the voltage supply system. In the power branch there are arranged two resistors connected in series with respect to one another, of which one can be actuated when the limiting threshold is exceeded for the purpose of a resistance reduction. During this process, the power that appears in the event of a malfunction is absorbed in the power branch. In the event of a malfunction, a very high power is converted [also: turned over ?] in the power branch because excess energy is produced in the voltage supply system when the voltage is high. For this reason, the disclosed protective device is only suitable to protect a circuit against very short voltage pulses.
However, if an overvoltage protective device is used in an onboard vehicle power supply system, conflicting demands must be met with respect to a high precision of the control system as well as to low costs of the components used for the control system with which excess power can be carried away if an overvoltage occurs. Overvoltages within a specific range must be tolerated, but if this range is exceeded, a protective circuit must respond reliably. This protective circuit, however, must not respond too early under any circumstances, namely within the permissible overvoltage range.
It is the object of the invention to provide a method and an arrangement for the overvoltage protection of an onboard vehicle power supply system having an energy store in order to protect the onboard vehicle power supply system reliably and with a high precision and, at the same time, economically against overvoltages in the onboard power supply system.